Off-Grid Solar-Powered Nanofiltration Pilot Study Relevant to The Navajo Nation

Abstract

Rural communities that are not connected to the electric or water grid often face challenges with access to affordable and safe potable water. Photovoltaic-powered membrane processes are being explored as an option to meet this need for cost-effective, safe drinking water in rural communities. In particular, the lower power requirements and palatable product water make nanofiltration an attractive option. The suitability of nanofiltration depends on many factors such as water quality, operating conditions, and maintenance. Commercially produced nanofiltration membranes are generally assumed to be operated near-continuously, but the nature of the solar power means that photovoltaic water filtration systems are operated intermittently, which may increase fouling and increase likelihood of microbial growth.Water quality analyses was conducted on at 17 sites on the Navajo Nation to determine the need for water treatment and the suitability of nanofiltration. Exceedances of water quality standards for arsenic and/or uranium were found at five sites and exceedances for total dissolved solids were found at 12 water sources, indicating the need for water treatment. Intermittent operation was tested on a pilot-scale nanofiltration prototype for two months with a salt solution with a total dissolved solids concentration of 1350 mg/L containing Mg2+, SO42+, Cl-, and Ca2+. There was little to no change in performance over the two months. Operation with a solution including ferric sulfate caused membrane fouling within 3 days. The system was also tested with a solution containing humic acid alone and a solution containing humic acid and Mg2+, SO42+, Cl-, and Ca2+ with a TDS of 1350 mg/L. The addition of salts decreased flux by about 30% when pH was maintained above 7.5 and about 12% when pH was not controlled. Finally, the membrane fouled by ferric iron was subjected to an autopsy. The results suggest the foulant layer consists primarily of amorphous, colloidal iron.